nsis: Allow installing app without admin previliges.
nsis: Allow installing app without admin previliges.

Allow the nsis installer to install the app to
AppData for a single user so that administrator
previliges are not required while installing.

Fixes #720.
zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-desktop